1. Field of the Invention
The present invention relates to an organic electroluminescence device, and more particularly, to a device from which light is emitted by applying an electric field on a thin film made of an organic compound.
2. Related Background Art
An organic electroluminescence device (hereinafter, simply referred to as an organic EL device) is a device that includes a thin film made of a fluorescence compound between an anode and a cathode, generates an exciton from the fluorescence compound by injection of an electron and an electron hole (hereinafter, also simply referred to as a hole) from each electrode, and uses light to be radiated when the exciton returns to the ground state.
The study conducted by Eastman Kodak Company in 1987 (Tang and SA Van Slyke, Appl. Phys. Lett., 51, p. 913, (1987)) reported light emission at about 1,000 cd/m2 by an applied voltage of about 10 V from a device including a function-separated two-layer structure having an anode made of ITO and a cathode made of magnesium-silver alloy where an aluminum quinolinol complex is used as each of an electron-transporting material and a light-emitting material and a triphenylamine derivative is used as a hole-transporting material. In this case, related patent documents include U.S. Pat. Nos. 4,539,507, 4,720,432, and 4,885,211.
In addition, light emission at spectra ranging from ultra-violet through infra-red can be allowed by changing the type of a fluorescence organic material. Recently, various compounds have been studied actively and described in many publications such as U.S. Pat. Nos. 5,151,629, 5,409,783, and 5,382,477, Japanese Patent Application Laid-Open Nos. H02-247278, H03-255190, H05-202356, H09-202878 and H09-2275756.
Furthermore, in addition to the organic EL devices using low molecular weight materials as described above, an organic EL device using a conjugated polymer has been reported from the group of the Cambridge University (Nature, 347, 539 (1990)). This report has confirmed light emission from a monolayer by film formation with polyphenylene vinylene (PPV) in a coating system. Patents related to an organic EL device using a conjugated polymer include U.S. Pat. Nos. 5,247,190, 5,514,878, and 5,672,678, Japanese Patent Application Laid-Open Nos. H04-145192 and H05-247460.
Recently, furthermore, an organic phosphorescence device using an iridium complex such as Ir (ppy)3 (Appl. Phys. Lett., 75, 4 (1999)) has been attracting attention and high luminous efficiency thereof has been reported.
Recent advances in organic EL devices are remarkable and the characteristics thereof allow the formation of light-emitting devices having high luminance with a low applied voltage, the variety of emission wavelengths, high-speed responsiveness, low profile, and lightweight, suggesting the possibility for extensive uses. However, organic EL devices still involve many problems in durability, such as chronological changes by prolonged use, and degradation with atmospheric gases containing oxygen, humidity, or the like. When applications of organic EL devices to full-color displays and so on are taken into consideration, under the present circumstances, blue-, green-, and red light-emissions with extended-life, high conversion rate, and high color purity have been demanded.
Examples of the materials and organic EL devices containing anthracene rings include a phenyl anthracene derivative disclosed in Japanese Patent Application Laid-Open No. H08-012600. In particular, when a phenyl anthracene derivative was used as a blue light-emitting material or an electron-injection transporting material, the phenyl anthracene derivative was believed to allow the formation of a good organic film because of its low crystallinity. However, the luminous efficiency and useful life of the phenyl anthracene ring were insufficient in practical application.
An aminoanthracene derivative and a diaminoanthracene derivative have been disclosed as other examples in Japanese Patent Application Laid-Open Nos. H09-157643 and H10-072579, respectively. In the documents, those materials were believed to generate green light-emission when they were used as light-emitting materials. However, devices prepared from those materials showed insufficient luminous efficiencies and their useful lives were still insufficient in practical application.
Japanese Patent No. 3008897 disclosed as another example a device using a particular bianthryl compound as a light-emitting material, which was believed to attain light emission with high luminance. However, the publication describes nothing about luminous efficiency and useful life.
Japanese Patent Application Laid-Open No. H11-008068 disclosed as still another example a device using a particular anthracene compound having an olefin portion as a light-emitting material, which was believed to attain light emission from yellow to red. However, the device showed insufficient luminous efficiency in practical application.
Furthermore, Japanese Patent Application Laid-Open No. 2001-284050 disclosed as another example a device that contains an anthracene derivative having a particular structure, an electron-transporting compound, and another fluorescence compound in a light-emitting medium layer. This device was believed to provide a red light-emitting device with improved reliability. However, the device showed insufficient luminous efficiency in practical application. In addition, it was difficult to obtain blue light emission because of its device configuration.